Thanks to Mehdi Dogguy, here's a nice hook to generate a source
change file at build time (with pbuilder), so one can upload
source-only packages to the archive and have buildds rebuild for
all the architectures. Put it in .pbuilder/hooks/B10_source-build
so it gets called once the builds succeeds

Next time you build a package, you should find, alongside the
<package>_<version>_<arch>.changes file, a
<package>_<version>_source.changes which you can use
with usual tools (lintian, debsign, dput…) to upload it to the
Debian archive.

Note that if you do that, you have to make sure that your
debian/rules support building separately the arch-dependent and
arch-independant packages. To check that, you can call pdebuild
like this: